rxin's comments.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/commands.scala

@@ -62,7 +62,16 @@ case class ExplainCommand(plan: LogicalPlan) extends Command {
 case class CacheCommand(tableName: String, doCache: Boolean) extends Command
 
 /**
- * Returned for the "Describe tableName" command.
+ * Returned for the "DESCRIBE tableName" command.
+ */
+
+/**
+ * Returned for the "DESCRIBE tableName" command.
+ * @param table The table to be described.
+ * @param isFormatted True if "DESCRIBE FORMATTED" is used. Otherwise, false.
+ *                    It is effective only when the table is a Hive table.
+ * @param isExtended True if "DESCRIBE EXTENDED" is used. Otherwise, false.
+ *                   It is effective only when the table is a Hive table.
  */
 case class DescribeCommand(
     table: LogicalPlan,

sql/core/src/main/scala/org/apache/spark/sql/execution/commands.scala

@@ -130,8 +130,10 @@ case class DescribeCommand(child: SparkPlan, output: Seq[Attribute])(
     @transient context: SQLContext)
   extends LeafNode with Command {
 
-  override protected[sql] lazy val sideEffectResult: Seq[(String, String, String)] =
-    child.output.map(field => (field.name, field.dataType.toString, None.toString))
+  override protected[sql] lazy val sideEffectResult: Seq[(String, String, String)] = {
+    Seq(("# Registered as a temporary table", null, null)) ++
+      child.output.map(field => (field.name, field.dataType.toString, null))
+  }
 
   override def execute(): RDD[Row] = {
     val rows = sideEffectResult.map {

sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveQl.scala

@@ -369,7 +369,7 @@ private[hive] object HiveQl {
 
   protected def extractDbNameTableName(tableNameParts: Node): (Option[String], String) = {
     val (db, tableName) =
-      tableNameParts.getChildren.map{ case Token(part, Nil) => cleanIdentifier(part)} match {
+      tableNameParts.getChildren.map { case Token(part, Nil) => cleanIdentifier(part) } match {
         case Seq(tableOnly) => (None, tableOnly)
         case Seq(databaseName, table) => (Some(databaseName), table)
       }
@@ -379,7 +379,8 @@ private[hive] object HiveQl {
 
   protected def nodeToPlan(node: Node): LogicalPlan = node match {
     // Just fake explain for any of the native commands.
-    case Token("TOK_EXPLAIN", explainArgs) if noExplainCommands contains explainArgs.head.getText =>
+    case Token("TOK_EXPLAIN", explainArgs)
+      if noExplainCommands.contains(explainArgs.head.getText) =>
       ExplainCommand(NoRelation)
     case Token("TOK_EXPLAIN", explainArgs) =>
       // Ignore FORMATTED if present.

sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveStrategies.scala

@@ -17,7 +17,7 @@
 
 package org.apache.spark.sql.hive
 
-import org.apache.spark.sql.{SQLContext}
+import org.apache.spark.sql.SQLContext
 import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.planning._
 import org.apache.spark.sql.catalyst.plans._
@@ -88,10 +88,6 @@ private[hive] trait HiveStrategies {
             Seq(DescribeHiveTableCommand(
               t, describe.output, describe.isFormatted, describe.isExtended)(context))
           case o: LogicalPlan =>
-            if (describe.isFormatted)
-              logger.info("Formatted is ignored because it is not defined for non-Hive tables.")
-            if (describe.isExtended)
-              logger.info("Extended is ignored because it is not defined for non-Hive tables.")
             Seq(DescribeCommand(planLater(o), describe.output)(context))
         }
       }

sql/hive/src/main/scala/org/apache/spark/sql/hive/execution/hiveOperators.scala

@@ -19,11 +19,11 @@ package org.apache.spark.sql.hive.execution
 
 import org.apache.hadoop.hive.common.`type`.{HiveDecimal, HiveVarchar}
 import org.apache.hadoop.hive.conf.HiveConf
-import org.apache.hadoop.hive.metastore.api.FieldSchema
 import org.apache.hadoop.hive.metastore.MetaStoreUtils
+import org.apache.hadoop.hive.metastore.api.FieldSchema
 import org.apache.hadoop.hive.ql.Context
-import org.apache.hadoop.hive.ql.metadata.formatting.MetaDataFormatUtils
 import org.apache.hadoop.hive.ql.metadata.{Partition => HivePartition, Hive}
+import org.apache.hadoop.hive.ql.metadata.formatting.MetaDataFormatUtils
 import org.apache.hadoop.hive.ql.plan.{TableDesc, FileSinkDesc}
 import org.apache.hadoop.hive.serde.serdeConstants
 import org.apache.hadoop.hive.serde2.objectinspector.ObjectInspectorUtils.ObjectInspectorCopyOption

sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveQuerySuite.scala

@@ -21,7 +21,9 @@ import scala.util.Try
 
 import org.apache.spark.sql.hive.test.TestHive
 import org.apache.spark.sql.hive.test.TestHive._
-import org.apache.spark.sql.{SchemaRDD, execution, Row}
+import org.apache.spark.sql.{SchemaRDD, Row}
+
+case class TestData(a: Int, b: String)
 
 /**
  * A set of test cases expressed in Hive QL that are not covered by the tests included in the hive distribution.
@@ -250,11 +252,11 @@ class HiveQuerySuite extends HiveComparisonTest {
     assert(Try(q0.count()).isSuccess)
   }
 
-  test("Describe commands") {
-    hql(s"CREATE TABLE test_describe_commands (key INT, value STRING) PARTITIONED BY (dt STRING)")
+  test("DESCRIBE commands") {
+    hql(s"CREATE TABLE test_describe_commands1 (key INT, value STRING) PARTITIONED BY (dt STRING)")
 
     hql(
-      """FROM src INSERT OVERWRITE TABLE test_describe_commands PARTITION (dt='2008-06-08')
+      """FROM src INSERT OVERWRITE TABLE test_describe_commands1 PARTITION (dt='2008-06-08')
         |SELECT key, value
       """.stripMargin)
 
@@ -267,20 +269,20 @@ class HiveQuerySuite extends HiveComparisonTest {
         Array("# Partition Information", null, null),
         Array("dt", "string", null))
     ) {
-      hql("DESCRIBE test_describe_commands")
+      hql("DESCRIBE test_describe_commands1")
         .select('name, 'type, 'comment)
         .collect()
     }
 
     // Describe a table with keyword FORMATTED
     // We only
     assertResult(6) {
-      hql("DESCRIBE FORMATTED test_describe_commands").count()
+      hql("DESCRIBE FORMATTED test_describe_commands1").count()
     }
 
     // Describe a table
     assertResult(6) {
-      hql("DESCRIBE EXTENDED test_describe_commands").count()
+      hql("DESCRIBE EXTENDED test_describe_commands1").count()
     }
 
     // Describe a table with a fully qualified table name
@@ -292,22 +294,22 @@ class HiveQuerySuite extends HiveComparisonTest {
         Array("# Partition Information", null, null),
         Array("dt", "string", null))
     ) {
-      hql("DESCRIBE default.test_describe_commands")
+      hql("DESCRIBE default.test_describe_commands1")
         .select('name, 'type, 'comment)
         .collect()
     }
 
     // Describe a column is a native command
     assertResult(Array(Array("value", "string", "from deserializer"))) {
-      hql("DESCRIBE test_describe_commands value")
+      hql("DESCRIBE test_describe_commands1 value")
         .select('result)
         .collect()
         .map(_.getString(0).split("\t").map(_.trim))
     }
 
     // Describe a column is a native command
     assertResult(Array(Array("value", "string", "from deserializer"))) {
-      hql("DESCRIBE default.test_describe_commands value")
+      hql("DESCRIBE default.test_describe_commands1 value")
         .select('result)
         .collect()
         .map(_.getString(0).split("\t").map(_.trim))
@@ -325,11 +327,29 @@ class HiveQuerySuite extends HiveComparisonTest {
         Array("", "", ""),
         Array("dt", "string", "None"))
     ) {
-      hql("DESCRIBE test_describe_commands PARTITION (dt='2008-06-08')")
+      hql("DESCRIBE test_describe_commands1 PARTITION (dt='2008-06-08')")
         .select('result)
         .collect()
         .map(_.getString(0).split("\t").map(_.trim))
     }
+
+    // Describe a registered temporary table.
+    val testData: SchemaRDD =
+      TestHive.sparkContext.parallelize(
+        TestData(1, "str1") ::
+        TestData(1, "str2") :: Nil)
+    testData.registerAsTable("test_describe_commands2")
+
+    assertResult(
+      Array(
+        Array("# Registered as a temporary table", null, null),
+        Array("a", "IntegerType", null),
+        Array("b", "StringType", null))
+    ) {
+      hql("DESCRIBE test_describe_commands2")
+        .select('name, 'type, 'comment)
+        .collect()
+    }
   }
 
   test("parse HQL set commands") {